This invention relates generally to underwater acoustic detection systems and more particularly to underwater Doppler detection systems.
Underwater Doppler detection systems have been used frequently in the past to determine the relative velocity between a source of acoustic energy and a target. When a beam of ultrasonic energy is projected into an inhomogeneous fluid, some of the energy is scattered back in the direction of the transmitter. If a relative motion exists between the fluid and the transmitter, the reflected signal will differ in frequency from the transmitted signal by: ##EQU1## where F.sub.D is the difference in frequencies, F.sub.T is the transmitted frequency, C is the speed of sound, and V is the magnitude of relative fluid velocity along a line normal to the transducer plane. If a separate receiver and transducer are used, the angle .theta. is the angle between the transducer axis and the transducer base line. A positive frequency shift occurs when the relative fluid velocity is toward the transducers, while a negative frequency shift occurs when the relative fluid velocity is away from the transducers.
While Doppler detectors have been utilized in the past to determine relative velocity between transmitter and target, prior art Doppler systems do not analize the vector gradient of the rotation of wakes and are thus incapable of detecting water turbulence produced by submarines.